Wireless communications has emerged to become a huge market as millions of people world-wide buy cellular handsets, subscribe to Personal Communications Services (PCS), and make calls on a daily basis. There are many competing technologies in the wireless communications field. Initially, cellular transmissions were made according to traditional analog radio frequency (RF) technology. But as wireless digital technology improved, it became clear that digital applications were far superior to that of analog. The three dominant wireless digital technologies existing today include Global System of Mobile communications (GSM), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). Of these three digital wireless technologies, CDMA is gaining widespread popularity because of its many advantages.
Generally, CDMA offers greater signal quality, resulting in clearer calls. In addition, CDMA utilizes a spread-spectrum approach, which makes it ideal for deployment in dense urban areas where multi-pathing is an issue. This results in fewer dropped calls. Furthermore, CDMA technology is more power efficient, thereby prolonging the standby and active battery life. But one of the most attractive features of CDMA is that it offers a greater capacity for carrying signals. Basically, the airwaves are divided into a number of different frequency bands per Federal Communications Commission (FCC) regulations. A limited segment of the airwaves has been allocated by the FCC for cellular usage. Due to the huge demand for cellular usage and the limited bandwidth that is available, getting a license from the FCC to transmit on a particular frequency band is extremely expensive. By increasing capacity, CDMA enables PCS providers to carry more users per channel. This increased capacity directly translates into greater revenue for cellular companies.
The advantages of CDMA carry over into high-speed wireless digital access. Increasingly, wireless digital applications are being used to access digital data (e.g., the Internet, intranet, multimedia, business data, etc.) at high speeds. With high speed wireless access, mobile users can obtain instant access to the Internet, business data (e.g., stock market quotes, sales reports, inventory information, price checks, customer data, emails, pages, etc.), and other real time data (e.g., traffic updates, weather information, sports news, etc.). The goal is to provide cellular handsets, personal digital assistants, portable communications devices, etc. the ability to transmit and receive digital data as well as make conventional telephone calls. The trend is towards ever faster mobile data speeds to meet customer demands. With greater data speeds, it is possible to provide even more data to more users. Recent CDMA based standards such as IS-95 and 3G are proposing increased data rates and capabilities.
Presently, virtually all CDMA technology entails using three separate modulation stages. FIG. 1 shows a typical prior art CDMA system. User signals (e.g., digitized voice signals or digital packetized data) are first modulated by a code which enables multiple users to share the same cell. The most commonly used code is known as a “Walsh” function. As stated above, one advantage of CDMA for personal communication services is its ability to accommodate many users on the same frequency at the same time. This is accomplished by assigning a specific “Walsh” code to each user. Only that particular code can demodulate the transmitted signal for that particular user. Since Walsh codes are orthogonal, users with different codes do not interfere with each other. Next, the signal is modulated by a pseudo-random number. This effectively serves to “spread” the transmitted signal across a wider spectrum. By spreading the signal out across a wider spectrum, the overall power of the transmitted signal can be boosted without exceeding the FCC regulations in any one channel. Finally, all users of that cell are summed and modulated by a sinusoidal carrier.
The universally accepted rational behind modulating the signal with a sinusoidal carrier is based on the theory which states that the length of the antenna should be proportional to the wavelength being transmitted. Following this conventional theory, it would be theoretically impossible to design an antenna large enough to efficiently transmit and receive baseband signals. As such, all modern direct sequence, spread spectrum CDMA systems uses a carrier. This extra modulation step adds complexity and incurs extra costs.
Thus, it would be beneficial if there were a way to eliminate one or more of the three separate modulation steps currently used in CDMA systems. Such an apparatus and method would be simpler, more reliable, and more cost efficient to produce.